Ammonia or urea may be stored on-board a vehicle for use as a reductant in exhaust emission control systems.
Under some conditions, such as during engine starting, ammonia vapor that was generated in the reductant system may be used, such as described in U.S. 2006/0117741. For example, ammonia vapors may be routed to a storage unit and a reactor that uses exhaust heat or electrically generated heat. Further, the vapors may also be directed to the exhaust gas to react with exhaust gas NOx in a catalyst in the exhaust flow.
However, the inventors herein have recognized several issues with such a system. For example, ammonia vapors may be generated during various increased temperature conditions, possibly increasing pressure in the system and enabling ammonia to escape to the atmosphere without being reacted in the exhaust. For example, after an engine shut-down and when the vehicle may be parked in an enclosed area, residual heat and ambient heat may substantially increase vapor pressure, and possibly escape un-metered from the reductant system, either through the exhaust or through other passages.
Thus, in one approach, a method of managing vapors generated from an ammonia-containing reductant delivery system for an engine of a vehicle may be used. The method comprises during at least a portion of engine-off conditions, storing ammonia containing vapors generated in the reductant delivery system, and after said storing and during at least a portion of engine operation, purging said stored ammonia into an exhaust of the engine to react in a catalyst in the exhaust flow.
In this way, it may be possible to reduce vapor pressures and reduce un-metered ammonia exiting the system. Further, by later use of the stored ammonia in the exhaust, more effective usage rates of ammonia (or urea) may be achieved, possibly reducing the amount or frequency of ammonia (or urea) that is added by a user.
Note that various forms of shut-down may be possible, such as temporary engine shut-downs due to vehicle stop-start operation, temporary engine shut-downs due to hybrid vehicle operation, or complete vehicle shut-downs.